| Hot-dip galvanizing is widely used to enhance the corrosion resistance of iron and steel products. During the process of hot-dip galvanization, a series of Fe-Zn intermediate phases would be formed near the steel surface because of the interaction between iron and molten zinc. The alloying elements in steel would affect the growth of galvanized coating, which has aroused the concern of many researchers. Zinc coatings have been alloyed with a number of elements to improve the corrosion resistance of the coating. ZAM coating, a newly developed galvanized coating contains 6 wt.% Al and 3 wt.% Mg, possesses excellent corrosion resistance. Because a small amount of primary MgZn2 phase there exist in ZAM coating, which is hard and brittle, the crack resistance of the coating is decreased obviously. Therefore, the further research on the growth of Fe-Zn alloy layer and microstructure of ZAM coating is of great importance to hot-dip galvanizing industry.Based on previous studies of our research group, according to the the fact that TiAl is the most stable phase in Al-Ti binary alloy(the mass ratio of Ti/Al is 2:1), the affecting mechanism of Ti in steel on the growth of Fe-Zn intermetical alloy layer in(Zn-0.2wt%Al)/(Fe-Ti) diffusion couple has been explained. Meanwhile, based on the experimental studies of our research group, the influence of P on the microstructure of Fe-P alloy has been investigated. Besides, the influence of P in steel substrate on the growth of Fe-Zn intermetical alloy layer in the Zn/Fe-P liquid-solid diffusion couple has been explained using diffusion channel model.In the present paper, it was found that that thick dendritic Al-rich phase in the surface structure is refined obviously by adding small amount of Si in ZAM bath, and the dendrites become finer with the increasing of Si content. The volume fraction of dendritic Al-rich phase falls and the volume fraction of Zn/Al/MgZn2 ternary eutectic structure rises with the increasing of Si content. At the same time, η-Zn and MgZn2 phase disappear, Mg2 Si particles grow in the surface structure by the addition of Si. Dipping pure iron specimens in Zn-6%Al-3%Mg-0.1Si bath for 60 s, dendritic Al-rich phase in the surface structure become thicker obviously and the volume fraction wane with the rising of dipping temperature, abnormal surface structure occurs when the dipping temperature is 530℃.In present paper, the experimental results show that a small amount of Ti in ZAM bath refines the grain of solidification structure and eliminate the coarse MgZn2 phase, thus the property of the ZAM coating is improved. When the Ti content in a Zn-6Al-3Mg bath is less than 0.053 wt.%, a subsequent increase of Ti in the bath causes a reduction in the average thickness of the resulting FeAl3 Znx layer and the Fe2Al5 Znx layer. The restraining effect of Ti on the growth of Fe2Al5 Znx layer is attributed to the occupation of Ti atoms in vacancies in the Fe2Al5 Znx phase. When the Ti content exceeds 0.053 wt.%, the average thickness of the FeAl3 Znx and Fe2Al5 Znx layers increases significantly. The effect of Ti on the growth of the Fe-Al alloy layer in hot-dip Zn-6Al-3Mg coatings can be explained by a diffusion path model. |
PDF Full Text Request